Grasslands Observatory - Essays - The Milky Way versus M31

The Milky Way has 11 known satellites [Table IA], the most important of which are the Large and Small Magellanic Clouds and the Sagittarius Dwarf Galaxy (Sparke 2000). The satellite galaxies of the Milky Way lie nearly in the same plane and may have formed out of a single gas cloud captured by the Milky Way (Sparke 2000). The Large Magellanic Cloud (LMC) has approximately 10% the luminosity of the Milky Way, and it measures 14 kpc in longest dimension. It is the prototype for the Sm class of Magellanic spirals. It is a distorted spiral galaxy with a bar, while the Small Magellanic Cloud (SMC) is ten times fainter and is an elongated cigar shaped ellipsoid structure seen end on (Sparke 2000). Both Magellanic clouds are rich in gas and show active star formation. A gaseous bridge connects the two galaxies, and a large gas stream, The Magellanic Stream, trails from the SMC, merges into the bridge between the Magellanic Clouds, and goes into a “Leading Arm” running to the Milky Way. The Magellanic Clouds orbit each other and orbit the Milky Way. They are on a plunging eccentric orbit around the Milky Way and made a close approach to the Milky Way 200-400 million years ago (Sparke 2000).

The Milky Way is aggressively disrupting the Magellanic Clouds, and at the same time is in the process of cannibalizing the Sagittarius Dwarf Galaxy (Mateo 2000). The SMC was probably significantly disrupted by its close encounter with the LMC and the Milky Way 200-400 million years ago (Spark 2000; Mateo 2000). The Magellanic Clouds are predicted to fall into the Milky Way in a few billion years and will be totally disrupted in the process (Mateo 2000).

M31 also has at least 11 satellite galaxies [Table IB]. These include two relatively prominent close galaxies, M32 (NGC221) and M110 (NGC205) [figure 1A]. M32 is a low luminosity elliptical dwarf galaxy, and M110 is a small elliptical galaxy. NGC147 and NGC185 are other dwarf elliptical galaxies which are satellites of M31. Other dwarf irregular or dwarf spheroidal galaxies that are satellites of M31 include IC10, LGS3, AndI, AndII, AndIII, AndV, and AndVI (Walterbos 2000). M110 is interacting with M31, which is distorting M110 and pulling at its outer stars. M32 has a very high central brightness, and it could be “a miniature version of a normal or ‘giant’ elliptical galaxy” (Sparke 2000). It may have a large black hole at its center and be the remnant of a much larger galaxy, perhaps a galaxy that underwent a past disruptive interaction with M31. M32’s distance from M31 is unknown, and its motion is not known well enough to determine if it has undergone a recent interaction with M31 (Sparke 2000). There is a giant stream of metal rich stars within the halo of M31. This stream could have M32 and M110 as its source. Both galaxies have lost a large number of stars due to tidal interactions with M31 (Ibata 2001).

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